DETAILED MAPPING OF LEUCOCRATIC BODIES IN THE PINKHAM NOTCH MIGMATITE, NEW HAMPSHIRE: DO LEUCOSOMES REPRESENT MELT PLUMBING?

Anatectic rocks in Pinkham Notch, NH (Mt. Washington area; Devonian, Central Maine belt) are among the best examples of stromatic migmatite on earth. Leucosome is ~ 20 vol.%, higher than usual <10% found in metatexites, including the rest of the Central Maine belt. We supposed two things. First, leucosome and other granite geometries could illustrate the melt plumbing. Alternatively, documentation could answer why leucosomes are in high % where melt escape would have been more likely. Why did these rocks record higher-% leucosome relative to regional metatexites?

Mapping was at the cm-m scale at a single 25x60 m pavement exposure in Peabody River at the Great Gulf trail head. The exposure was divided into 53 sq. meters, gridded parallel to structure (NE-SW) and used for orthographic projection. Data collected at each block included mapping leucosome contacts, structural element orientations, and mineral and textural composition of all granitic components. The surface orientation was recorded for the 3-D projection of the exposure. Leucosomes are trondjemitic (Pl, Qtz, Ms) with an overall milky-white color. Cm-size Qtz and Ms spangles are found in the larger granitic units that include clustered Grt. Mica spangles are clusters throughout the granitic material, whereas Qtz is clustered elsewhere. Granitic bodies are grouped based on diameter and texture: <3 mm, 3-5 mm, 5-10 mm, >10 mm. The largest are granitic sills. Contacts between the leucosome and melanosomes are sharp. Many bodies of each group are boudinage and/or foldsd. Each group shows distinct textures (increasing grain size), but similar mineral compositions. The stromatic structure of the rock is oriented NE-SW in the outcrop as a whole, but is folded asymmetrically at the meter scale. The largest granites are sub-concordant with the leucosomes (also folded), whereas two larger granitic dykes moderately cross-cut (E-W). Based on these data, the stromatic structure has recorded solid-state strain when the rock was sub-solidus. However, during migmatite formation the strength of the rock would have been compromised given the vol.% granite body if these were all molten at the same time. Therefore, we suggest this was not the case, and each body group represents both an averaging of melt presence, and a separate time of migmatite formation recorded within this rock.